Using avios to save on cash tickets with the cash+avios option will get you roughly $80 per 1000 avios up to 20 000 avios then a slowly decreasing value down to roughly $70 per 1000 avios when using avios to pay 50% of the ticket.
So there isn’t any reason to buy avios (sales are always over $100 per 1000 avios) for cash+avios tickets.